Optical waveguides are designed to guide light of various modes and polarization states contained within a range of wavelengths in a controlled fashion. Single-mode optical fiber is the most common waveguide for long-distance delivery of light. Other waveguides, such as diffused waveguides, ion-exchanged waveguides, strip-loaded waveguides, planar waveguides, and polymer waveguides are commonly used for guiding light over short distances and especially for combining or separating light of different wavelengths, optical frequency mixing in nonlinear optical materials, modulating light and integrating many functions and operations into a small space.
In essence, a waveguide is a high refractive index material, usually referred to as the core in an optical fiber, immersed in a lower index material or structure, usually referred to as the cladding, such that light injected into the high index material within an acceptance cone is generally confined to propagate through it. The confinement is achieved because at the interface between the high and low index materials the light undergoes total internal reflection (TIR) back into the high index material.
The problem of amplifying optical signals for long distance transmission was successfully addressed by the development of Erbium doped fiber amplifiers (EDFAs). An EDFA consists of a length of silica fiber with the core doped with ionized atoms (Er3+) of the rare earth element Erbium. The fiber is pumped with a laser at a wavelength of 980 nm or 1480 nm. The doped, pumped fiber is optically coupled with the transmission fiber so that the input signal is combined with the pump signal in the doped fiber. An isolator is generally needed at the input and/or output to prevent reflections that would convert the amplifier into a laser. Early EDFAs could provide 30 to 40 dB of gain in the C-band extending between 1530 to 1565 nm with noise figures of less than 5 dB. Recently, EDFAs have been developed that can provide 25 dB of gain in the L-band (1565 to 1625 nm) as well as in the C-band.
There is great interest in the telecommunications industry to make use of the optical spectrum range with wavelengths shorter than those currently achievable with conventional C-band and L-band EDFAs. This wavelength range, commonly called the “S-band” or “short-band” is poorly defined because there is no consensus on the preferred amplifier technology. In general, however, the S-band is considered to cover wavelengths between about 1425 nm and about 1525 nm.
The gain in the S-band typically observed in EDFAs is limited by several factors, including incomplete inversion of the active erbium ions and by amplified spontaneous emissions (ASE) or lasing from the high gain peak near 1530 nm. Unfortunately, at present no efficient mechanism exist for suppressing ASE at 1530 nm and longer wavelengths in an EDFA.
At this time, the prior art offers various types of waveguides and fibers in which an EDFA can be produced. It is therefore useful to briefly review prior art fibers and waveguides.
Most waveguides are designed to prevent injected light from coupling out via mechanisms such as evanescent wave out-coupling (tunneling), scattering, bending losses and leaky-mode losses. A general study of these mechanisms can be found in the literature such as L. G. Cohen et al., “Radiating Leaky-Mode Losses in Single-Mode Lightguides with Depressed-Index Claddings”, IEEE Journal of Quantum Electronics, Vol. QE-18, No. 10, October 1982, pp. 1467-72. In this reference the authors describe the propagation of light in more complex lightguides with claddings having a variation in the refractive index also referred to as depressed-clad fibers.
L. G. Cohen et al. teach that varying the cladding profile can improve various quality parameters of the guided modes while simultaneously maintaining low losses. Moreover, they observe that depressed-index claddings produce high losses to the fundamental mode at long wavelengths. Further, they determine that W-profile fibers with high index core, low index inner cladding and intermediate index outer cladding have a certain cutoff wavelength above which fundamental mode losses from the core escalate. These losses do not produce very high attenuation rates and, in fact, the authors study the guiding behavior of the fiber near this cutoff wavelength to suggest ways of reducing losses.
U.S. Pat. Nos. 5,892,615 and 6,118,575 teach the use of W-profile fibers similar to those described by L. G. Cohen, or QC fibers to suppress unwanted frequencies and thus achieve higher output power in a cladding pumped laser. Such fibers naturally leak light at long wavelengths, as discussed above, and are more sensitive to bending than other fibers. In fact, when bent the curvature spoils the W or QC fiber's ability to guide light by total internal reflection. The longer the wavelength, the deeper its evanescent field penetrates out of the core of the fiber, and the more likely the light at that wavelength will be lost from the core of the bent fiber. Hence, bending the fiber cuts off the unpreferred lower frequencies (longer wavelengths), such as the Raman scattered wavelengths, at rates of hundreds of dB per meter.
Unfortunately, the bending of profiled fibers is not a very controllable and reproducible manner of achieving well-defined cutoff losses. To achieve a particular curvature the fiber has to be bent, e.g., by winding it around a spool at just the right radius. Different fibers manufactured at different times exhibit variation in their refractive index profiles as well as core and cladding thicknesses. Therefore, the right radius of curvature for the fibers will differ from fiber to fiber. Hence, this approach to obtaining high attenuation rates is not practical in manufacturing.
In producing an EDFA for the S-band the relatively high losses and low gains over the S-band render the selection of fiber and fiber profile even more difficult. In fact, the problems are so severe that the prior art teaches interposition of external filters between EDFA sections to produce an S-band EDFA. For example, Ishikawa et al. disclose a method of fabricating an S-band EDFA by cascading five stages of silica-based EDFA and four ASE suppressing filters in Ishikawa et al., “Novel 1500 nm-Band EDFA with discrete Raman Amplifier”, ECOC-2001, Post Deadline Paper. In Ishikawa et al.'s experimental setup, the length of each EDA is 4.5 meters. The absorption of each suppressing filter at 1.53 μm is about 30 dB and the insertion losses of each suppressing filter at 1.48 μm and 0.98 μm are about 2 dB and 1 dB respectively. The pumping configuration is bi-directional, using a 0.98 μm wavelength to keep a high inversion of more than D≧0.7 (D, relative inversion). The forward and backward pumping powers are the same and the total pumping power is 480 mW. Ishikawa et al. show a maximum gain of 25 dB at 1518.7 nm with 9 dB gain tilt.
This method is relatively complicated and not cost-effective, as it requires five EDFAs, four ASE suppressing filters and high pump power. Also, each of the ASE suppressing filters used in Ishikawa et al.'s method introduces an additional insertion loss of 1-2 dB. The total additional insertion loss is thus about 4-8 dB.
In view of the above, it would be an advance in the art to provide a fiber amplifier exhibiting net gains over the S-band and not requiring external filters and having low pump power requirements. Specifically, it would be an advance to provide an EDFA with distributed ASE suppression in the C-band and L-band or substantially at 1530 nm and longer wavelengths over the whole length of a fiber amplifier. It would be also a welcome advance in the art to provide a method of designing such fiber amplifiers with net gain over the S-band.